There is an ongoing need to purify particles, both directly upon their synthesis and for the purposes of reusing and recycling them. Moreover, working with purified particles can be critical when conducting fundamental studies of their properties, since the purity of particles may be strongly correlated with their observed functional properties, as well as their stability. (See, for example, Kalyuzhny et al., “Ligand Effects on Optical Properties of CdSe Nanocrystals”, The Journal of Physical Chemistry B, 2005, 109(15), pp. 7012-7021.) Common impurities include excess ligands and stabilizers, small organic and inorganic molecules, and residual solvent. The preparation of high purity nanoparticles (e.g., materials having at least one characteristic dimension between 0.5 nm and 1000 nm) can be especially difficult. Such materials must often be separated from residual reactants, excess ligands, solvents, and other materials present in the raw reaction solution.
Various techniques have been developed to address nanoparticle purification, but all suffer from shortcomings. The most common technique for nanoparticle purification is the precipitation-dissolution technique. (See, for example, Aldana et al., “Photochemical Instability of CdSe Nanocrystals Coated by Hydrophilic Thiols”, Journal of the American Chemical Society, 2001, 123(36), pp. 8844-8850; Qu et al., “Alternative Routes toward High Quality CdSe Nanocrystals”, Nano Letters, 2001, 1(6), pp. 333-337; and Yu et al., “Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals”, Chemistry of Materials, 2003; 15(14), pp. 2854-2860.) In this technique, a so-called nonsolvent is added to a colloidal nanoparticle solution until agglomeration or precipitation of the nanoparticles occurs. This technique involves a number of steps, however, which must be repeated, so that significant amounts of time and material (e.g., solvents) are required.
Dialysis is also used to separate particles. (See, for example, Xie et al., “Dendrimer-mediated synthesis of platinum nanoparticles: new insights from dialysis and atomic force microscopy measurements”, Nanotechnology, 2005(7), p. S492.) A solution is placed in a membrane impermeable to the particles and then dipped into a pure solvent bath. The impurities pass through the membrane and into the solvent bath, which is refreshed from time to time. This process is very time consuming, since it relies on diffusion, and it can require a significant amount of solvent. This technique is especially problematic for purifying nanoparticles, since membrane materials with controlled pore size on the nanoscale are expensive.
Purification by ultra-filtration is achieved by passing a solution through a filter having very small pores that do not allow for the passage of the particles. (See, for example, Weng et al., “Exploring Feasibility for Application of Luminescent CdTe Quantum Dots Prepared in Aqueous Phase to Live Cell Imaging”, Chinese Chemical Letters, 2006, 17(5), pp. 675-678.) This process becomes slower and more energy intensive as the particle size decreases, so that materials having a controlled pore size on the nanoscale, especially below 20 nm, are expensive to fabricate.
Diafiltration has also been used for purifying nanoparticles. (See, for example, Sweeney et al., “Rapid Purification and Size Separation of Gold Nanoparticles via Diafiltration”, Journal of the American Chemical Society, 2006, 128(10), pp. 3190-3197; and Bianchi et al., “Purification of Nanoparticles by Hollow Fiber Diafiltration”, in Nanotechnology 2008: Microsystems, Photonics, Sensors, Fluidics, Modeling and Simulation, Editor: Nano Science & Technology Institute, Cambridge, Mass., USA, Oct. 13, 2008.) Nanoparticles are separated from molecular impurities based on their larger size, in this case, by flowing the nanoparticles through a tubular nanoporous membrane. Membranes of this type are generally expensive and can suffer from fouling and clogging during use.
Size exclusion chromatography has also been used for the purification of colloidal nanoparticles. (See, for example, Wang et al., “Preparative size-exclusion chromatography for purification and characterization of colloidal quantum dots bound by chromophore-labeled polymers and low-molecular-weight chromophores”, Journal of Chromatography A, 2009, 1216(25), pp. 5011-5019.) In this technique, colloidal solutions are passed through a porous stationary phase (e.g., Bio-Beads®). Smaller impurities, due to the longer accessible path in the pores, remain entrained in the column longer while the particles are passed more rapidly. Specific functionality can be added to the column to increase the residence time of the impurities. High-performance liquid chromatography has been used as well to separate nanoparticles (see Wang et al., ibid.). A solution containing the particles is passed through a packed column under high pressure. Copious amounts of solvents relative to purified product are used to elute the injected materials.
Dielectrophoretic electrode arrays have also been used for the separation and manipulation of particles. (See, for example, Li et al., “Analysis of dielectrophoretic electrode arrays for nanoparticle manipulation”, Computational Materials Science, 30, 2004, pp. 320-325; and Green et al., “Dielectrophoretic separation of nano-particles”, Journal of Physics D: Applied Physics, 30, 1997, L41-44.) An AC electric field (that is not spatially uniform) is applied to a collection of polarizable particles, such that the particles experience a translational force that depends both on the relative polarizability of the particles and the applied frequency. Specifically, particles whose polarizability is greater than that of the host medium tend to congregate where the electric field is strongest, whereas particles whose polarizability is less than that of the host medium tend to congregate where the electric field is weakest. Unfortunately, this technique does not lend itself to bulk processing.
Accordingly, the disadvantages of existing purification techniques become acute as the particle size is reduced to the nanometer scale, and as the size, weight, and solubility of the nanoparticles approach those of the impurities being removed. In addition, most current purification methods require significant labor, time, and materials (especially solvents), thereby increasing the cost and the environmental impact of the purification process.